This invention relates to a PRI (pulse repetition interval) autocorrelation system responsive to pulse time of arrival data received from a plurality of emitters and particularly to an improved PRI autocorrelation system that operates at a high speed, provides highly accurate processing and detects emitters substantially without the erroneous effects of incomplete time of arrival data sets.
PRI autocorrelation is a technique for mapping relative activity versus PRI of a data sample formed as time of arrival (TOA) digital words marking the changing occurrence of pulse inputs received by a receiver system from a plurality of emitters. The data map of activity versus PRI is used to determine the most likely PRI or PRIs at which emitters might be found. The PRI values each derived from a specific emitter are then utilized for emitter identification, for tracking or for further processing. The electronic warfare defense systems in which PRI values are utilized are discussed, for example, in Electronic Warfare, Vol. 5, No. 4, August 1973, pp. 62-65 which discusses an EW (Electronic Warfare) system in which alphanumerical displays are provided showing signal frequency and pulse repetition frequencies as well as other parameters. Also a system is discussed in the Electronic Warfare, Vol. 10, No. 1, January 1978, in an article entitled "EW Defense Electronics" in which an ESM system developed by Deca Radar, Hersham, England is discussed. Conventional techniques for PRI identification involve computation of the time difference between data samples and picking the difference with the greatest number of occurrences.
Because of the large amount of data that can be generated in the course of time differencing, relatively few samples are used to make a decision and thus mistakes in the determination of the presence of an emitter may occur. Typically TOA differencing algorithms select only a single difference and seek to confirm it to save time. Thus, differencing systems are highly susceptible to missing pulses in the data sets, resulting in matching of harmonics rather than the fundamentals and erroneous indication of the presence of emitters. Other prior art techniques of PRI data mapping for identification, involve fast Fourier transforms, chirp Z transforms and Walsh transform methods, all of which have the limitations that they are substantially unable to identify intermittent pulse groups because of the fixed time record length requirements of these techniques. Also, all of the above mentioned prior art concepts are relatively complex and relatively slow in operation.